Do the B&T Mp5k stocks "lock" in the folded position? I've handled a friend's Choate and aside from the wobble, the other turn off was the stock would flop open if the gun is turned on its side with stock in closed position. Nitpicky I realize, I just didn't like it.

No they do not lock folded

They do not lock closed, but they aren't loose either (at least mine are not)

Neither my Choate nor B&T stocks flop open. I don't see how a Choate could flop open; it's spring loaded.

The Choate stock has a wedge and spring, which give it a mechanical bias to remain in the folded position. They generally will not start to open if held sideways with the stock down, but will move if shaken. This also allows for one-handed deployment of the stock (without actually touching it) if you create enough centrifugal force to swing it open. It does, of course, lock in the open position (as we all probably know). 

Either stock can also have some paracord tied to it which will unfold it upon deployment from the Eagle Industries (HK) shoulder rig/holster. 

My B&T flopped open.  I tightened the Allen screw a bit and fixed it right up.

I never realized they could be tightened. The one I got stays solidly in place in folded position. But it's nice to know it can be tightened.